Using isotopically specified fluids as optical elements

ABSTRACT

Fluidic optical elements and systems use isotopically specified fluids for processing light passing therethrough. The isotopic composition of the fluid may be adjusted to vary the optical properties. The properties of the isotopically specified fluid may be monitored and adjusted to obtain the desired optical characteristics of the fluidic optical element. In one embodiment, a method of optically processing light includes directing light through an optical element that includes an isotopically specified fluid disposed in a confined space. The isotopically specified fluid is selected to provide a preset desired effect on the light directed therethrough for optically processing the light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/311,247filed Dec. 20, 2005, which in turn is a continuation of InternationalApplication No. PCT/US2004/021159 filed Jun. 30, 2004, which claims thebenefit of U.S. Provisional Patent Application No. 60/484,276 filed Jul.1, 2003. The disclosures of these applications are incorporated hereinby reference in their entireties.

BACKGROUND

The invention relates generally to optical systems and elements and,more particularly, to the use of isotopically specified, fluids asoptical elements including applications in immersion lithography.

An exposure apparatus is one type of precision assembly that is commonlyused to transfer images from a reticle onto a semiconductor wafer duringsemiconductor processing. A typical exposure apparatus includes anillumination source, a reticle stage assembly that retains a reticle, anoptical assembly (sometimes referred to as a projection lens), a waferstage assembly that retains a semiconductor wafer, a measurement system,and a control system. The resist coated wafer is placed in the path ofthe radiation emanating from a patterned mask and exposed by theradiation. When the resist is developed, the mask pattern is transferredonto the wafer. In microscopy, extreme ultraviolet (EUV) radiation istransmitted through a thin specimen to a resist covered plate. When theresist is developed, a topographic shape relating to the specimenstructure is left.

Immersion lithography is a technique that can enhance the resolution ofprojection lithography by permitting exposures with numerical aperture(NA) greater than one, which is the theoretical maximum for conventional“dry” systems. By filling the space between the final optical elementand the resist-coated target (i.e., wafer) with an immersion fluid,immersion lithography permits exposure with light that would otherwisebe totally internally reflected at an optic-air interface. Numericalapertures as high as the index of the immersion liquid (or of the resistor lens material, whichever is least) are possible. Liquid immersionalso increases the wafer depth of focus, i.e., the tolerable error inthe vertical position of the wafer, by the index of the immersion liquidcompared to a dry system with the same numerical aperture. Immersionlithography thus has the potential to provide resolution enhancementequivalent to the shift from 248 to 193 nm. Unlike a shift in theexposure wavelength, however, the adoption of immersion would notrequire the development of new light sources, optical materials, orcoatings, and should allow the use of the same or similar resists asconventional lithography at the same wavelength. In an immersion systemwhere only the final optical element of the optical assembly and itshousing and the wafer (and perhaps the stage as well) are in contactwith the immersion fluid, much of the technology and design developedfor conventional tools in areas such as contamination control, carryover directly to immersion lithography.

The immersion fluid in an immersion lithography system serves as afluidic optical element. Fluids can also be used to form opticalelements such as those inside the optical assembly of an exposureapparatus. The use of a fluid as an optical element raises certainissues and challenges but also provides new opportunities.

SUMMARY

Embodiments of the invention are directed to fluidic optical elementsand systems using isotopically specified fluids for processing lightpassing therethrough. The isotopic composition of the fluid may beadjusted to vary the optical properties. The properties of theisotopically specified fluid may be monitored and adjusted to obtaindesired optical characteristics of the fluidic optical element. Examplesof such properties include the isotopic composition, index ofrefraction, temperature, and pressure. Water has optical properties thatare suitable as the immersion fluid in an immersion lithography system.Water further has mechanical properties such as low viscosity andsurface tension characteristics that render it particularly desirablefor use in immersion lithography. The index of refraction of animmersion fluid plays a role in the imaging during lithography.Generally, an immersion fluid having a higher index of refraction ismore desirable. The heavier isotopes of water can provide a desiredhigher index of refraction.

In accordance with an aspect of the invention, a method of opticallyprocessing light comprises directing light through an optical elementthat includes an isotopically specified fluid disposed in a confinedspace. The isotopically specified fluid is selected to provide a presetdesired effect on the light directed therethrough for opticallyprocessing the light.

In some embodiments, the isotopically specified fluid comprises one ormore isotopes of a fluid. The isotopically specified fluid may comprisea plurality of isotopes of a fluid, and an isotopic composition of thefluid is monitored. The method further comprises adjusting amounts ofthe plurality of isotopes of the fluid to change the isotopiccomposition based on the monitored isotopic composition of the fluid.The method may also comprise adjusting amounts of the plurality ofisotopes of the fluid to form a variable isotopic composition of thefluid to provide variable optical characteristics of the opticalelement.

In specific embodiments, the isotopically specified fluid comprises oneor more isotopes of water. The method further comprises positioning theoptical element in an immersion lithography apparatus at a locationbetween an optical assembly and a substrate to be processed bylithography, wherein the light is directed through the optical assemblyand the optical element to the substrate. The isotopically specifiedfluid of the optical element has a first index of refraction and theconfined space is provided in a container comprising a material having asecond index of refraction that is different from the first index ofrefraction. The method further comprises recirculating the isotopicallyspecified fluid between the confined space of the optical element and arecirculation reservoir. The method may also comprise controlling atleast one of a temperature, a pressure, and an isotopic composition ofthe isotopically specified fluid in the optical element. The confinedspace is provided in a container which comprises fused silica. The lighthas a wavelength of about 193 nm or greater.

In accordance with another aspect of the invention, an optical elementcomprises a container and an isotopically specified fluid disposed inthe container. The isotopically specified fluid is selected to provide apreset desired effect on a light directed therethrough for opticallyprocessing the light.

In accordance with another aspect of the present invention, an opticalsystem for processing a substrate comprises an optical assembly spacedfrom a substrate by a space and an optical element disposed in the spacebetween the optical assembly and the substrate. The optical elementincludes an isotopically specified fluid selected to provide a presetdesired effect on a light directed therethrough for optically processingthe light.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements and in which:

FIG. 1 is a simplified schematic view of an optical assembly includingan optical element having an isotopically specified fluid according toan embodiment of the invention;

FIG. 2 is an elevational view of an optical element having anisotopically specified fluid according to another embodiment of theinvention;

FIG. 3 is a simplified schematic view of a fluidic system for an opticalelement illustrating a recirculation arrangement according to anotherembodiment of the invention;

FIG. 4 is a simplified schematic view of a fluidic system for an opticalelement illustrating an open system arrangement according to anotherembodiment of the invention;

FIG. 5 is a simplified schematic view of an immersion lithography systememploying an isotopically specified fluid according to anotherembodiment of the invention; and

FIG. 6 is a plot of refractive index versus D₂O concentration in waterbased on calculations.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an optical assembly 10 including two solid optical elements12, 14, and a fluidic optical element 16 disposed therebetween. In thespecific embodiment shown, the solid optical elements 12, 14 areconverging lenses and the fluidic optical element 16 is a diverginglens. The solid optical elements 12, 14 may be made of glass or thelike. The fluidic optical element 16 includes an isotopically specifiedfluid 20 disposed in a confined space defined by the surfaces of the twosolid optical elements 12, 14 and a side wall 18. The side wall 18 issealed to the solid optical elements 12, 14. The side wall 18 may bemade of a transparent material such as glass.

As used herein, the term “isotopically specified fluid” refers to afluid of defined chemical composition, where the molecules making up thefluid are distinguished by the specific isotopes of their elementalconstituents, the corresponding molecular weight, and theirconcentration in the fluid. For example, for the case of water, thebasic molecular constituent is H₂O and the fluid would be characterizedby information similar to the following: TABLE 1 Molecular Molecularconstituents weight Concentration Comments H₂O¹⁶ 18 0.99985 H = mostabundant isotope of hydrogen; O¹⁶ = most abundant isotope of oxygenHDO¹⁶ 19 <10⁻⁵ D = deuterium, heavy isotope of hydrogen D₂O¹⁶ 20 0.00015H₂O¹⁷ 19 <10⁻⁵ HDO¹⁷ 20 <10⁻⁵ D₂O¹⁷ 21 <10⁻⁵ H₂O¹⁸ 20 <10⁻⁵ HDO¹⁸ 21<10⁻⁵ D₂O¹⁸ 22 <10⁻⁵

Only stable isotopes are listed in Table 1. Radioactive isotopes, forthe case of water, are short lived enough that they are not likely to bepresent in a high enough concentration to be significant. In otherfluids however, radioactive isotopes may have to be considered as well.Table 1 is representative of “normal water” which contains approximately0.015% heavy water. The concentrations labeled “<10⁻⁵” represent levelsbelow which no significant change in refractive index occurs. In thecase of water, a change of approximately 10⁻⁵ may change the refractiveindex by about 1 ppm which is assumed to be insignificant for thepresent application. For other fluids and applications the level ofsignificant concentration would have to be determined.

FIG. 2 shows another fluidic optical element 30 including anisotopically specified fluid 32 disposed in a container 34. Thecontainer 34 may be made of glass, fused silica, or the like. In somecases, the isotopically specified fluid 32 has a first index ofrefraction and the material of the container 34 has a second index ofrefraction that is different from the first index of refraction. Theisotopically specified fluid 32 may include a plurality of isotopes of afluid. Different isotopes have different optical properties such as theindex of refraction. The amounts of the plurality of isotopes of thefluid may be adjustable to form a variable isotopic composition of thefluid to provide variable optical characteristics of the opticalelement. In addition to the isotopically specified fluid, the shape ofthe container 34, the optical properties of the container material, andthe configuration of the cavity for the isotopically specified fluid 32in the container 34 can be selected to produce the desiredcharacteristics of the fluidic optical element 30.

The fluidic optical element 30 has two materials, namely, the materialof the isotopically specified fluid 32 and the material of the container34. This makes it possible to correct for chromatic aberrations orspectral dispersions. Some prior optical elements rely on theintroduction of calcium fluoride into fused silica or a similar glassmaterial to form a solid optical element. While calcium fluorideprovides suitable optical properties, it is difficult and expensive toproduce due to its crystalline nature and it introduces certainundesirable material properties such as intrinsic birefringence, as wellas additional birefringence arising from mechanical stress in theoptical element. The isotopically specified fluid 32 in the fluidicoptical element 30 may replace the use of calcium fluoride in some prioroptical elements.

FIG. 3 shows a system 40 for providing a fluidic optical element 42 toprocess light passing therethrough. In this closed system, theisotopically specified fluid is recirculated between the optical element42 and a reservoir 44. One or more sensors 48 are used to monitor theproperties of the isotopically specified fluid, such as the isotopiccomposition, index of refraction, temperature, pressure, and the like.In FIG. 3, the sensors 48 are coupled to the reservoir 44, but may becoupled to the optical element 42 or another part of the recirculationsystem to monitor the fluid. The sensor information is provided from thesensors 48 to a controller 50, which adjusts the properties of theisotopically specified fluid based on the sensor information, such ascontrolling any of the temperature, pressure, index of refraction, andisotopic composition of the fluid.

FIG. 4 shows an open system 60 for providing a fluidic optical element62 to process light passing therethrough. The isotopically specifiedfluid is supplied to the optical element 62 from a source 64 and exitsthe optical element 62 to a drain 66. One or more sensors 68 are coupledto the optical element 62 or another part of the flow path upstream ordownstream of the optical element 62, and are used to monitor theproperties of the isotopically specified fluid. The sensor informationis provided from the sensors 68 to a controller 70 which adjusts theproperties of the isotopically specified fluid based on the sensorinformation.

The closed and open flow systems of FIGS. 3 and 4 maintain a constantflow of the isotopically specified fluid through the fluidic opticalelements. The flow can ensure uniformity of the isotopically specifiedfluid and properties of the optical elements, and can eliminate theformation of voids or the like. It can also facilitate monitoring andcontrol of the isotopically specified fluid for the optical elements.

FIG. 5 shows an immersion lithography system 110 including a reticlestage 112, a projection lens or optical assembly 114, and a wafer orsubstrate 116 supported on a wafer stage 118. An immersion apparatus 120is disposed between the final optical element 122 of the projection lens114 and the wafer 116 to provide an immersion fluid therebetween. In thepresent embodiment, the immersion fluid is an isotopically specifiedfluid, and forms a fluidic optical element in the path of the lightbetween the projection lens 114 and the wafer 116. The isotopicallyspecified fluid is typically provided as a continuous flow through thespace between the projection lens 114 and the wafer 116 to form thefluidic optical element.

The system for providing the isotopically specified fluid to theimmersion lithography system 110 is preferably a closed immersion fluidcontrol system, where the fluid is recovered and recycled after exposureof a wafer. This is highly desirable because it is important to maintainthe isotopic composition and thus the fluid's optical properties, andbecause at least some of the isotopes are likely to be rare andexpensive. Even the vapor from the fluid should be recovered. A systemwith many of the desired characteristics is described in PCT ApplicationNo. PCT/US04/10055, entitled Environmental System Including VacuumScavenge For An Immersion Lithography Apparatus, filed Mar. 29, 2004.Additional components may be used to ensure the purity and/or opticalproperties of the fluid. Some change in isotopic composition may occurfrom small differences in chemical reactivities or physical propertiesamong the isotopes. Also some impurities may be introduced into thefluid. The chemical and isotopic composition can be monitored, forexample, using either optical or mass spectroscopy. Alternatively, theoptical properties such as the refractive index can be monitored using arefractometer, and appropriate isotopic molecular constituents added tomaintain the property at a constant value. Since impurities may affectthe refractive index, this method may maintain a constant index eventhough the basic chemical composition of the fluid is changing.

The isotopically specified fluid is selected to provide a preset desiredeffect on a light directed therethrough for optically processing thelight, for example, to perform a lithography procedure. The isotopicallyspecified fluid may include a plurality of isotopes of a fluid. Thefluid should not react with surrounding components such as the containerof the fluidic optical element, the neighboring optical components, andsubstrates with which the fluid has contact. Water is particularlysuitable for certain applications in lithography. Water is generallystable and nonreactive with other optical components and substrates. Ifwater forms part of the optics in a system such as a lithography system,the optical properties of water will need to be known to almost ppmaccuracy. The optical properties of water are being measured.

Water may play an important role in immersion lithography. Water hasvery low optical absorption at wavelengths as short as 193 nm which isthe wavelength of an ArF beam. Water has a relatively low viscosity.Thus wafer stage motion and vibrations may not couple through the waterto the projection lens to an unacceptable extent. It is important tomonitor the isotopic purity or composition of water, because thedifferent isotopes can have significantly different optical properties.First, different sources of water may have slightly different isotopiccompositions, leading to possibly significant differences in opticalproperties. Second, the index of refraction of water can be variedsignificantly by varying the isotopic composition, which may allow someamount of tuning of the projection optics design. Third, water tuned toan appropriate index of refraction may be introduced elsewhere in theprojection optics, to tune the optics further. Because the opticaldispersion of water differs from that of fused silica, some amount ofchromatic correction is possible.

The refractive indices of the water isotopes are not well determined atthe short wavelengths used in optical lithography. The refractiveindices for the various isotopes of water are estimated here from theLorentz-Lorenz model of molecular polarizability:(n ²−1)/(n ²+2)=Aρ,

where n is the index of refraction, ρ is the density, and A is anempirical constant. If one assumes that A is the empirical constant forordinary water, the isotope effect arises basically from changes in thedensity. Ordinary water is presumed to be H₂ ¹⁶O. Normal “heavy” waterD₂O is taken to be D₂ ¹⁶O, where D is deuterium (²H), the heavy isotopeof hydrogen. Of course, there are other isotopes including HDO andheavier isotopes of oxygen, ¹⁷O and ¹⁸O, that will add to the densityeffect. At a temperature of 30° C. for a wavelength of 193 nm, theconstant A for ordinary water is about 0.262936. Table 2 shows thecalculated results of the index of refraction for different isotopes ofwater. The index of refraction of an immersion fluid plays a role in theimaging during lithography. Generally, an immersion fluid having ahigher index of refraction is more desirable. The heavier isotopes ofwater may provide the desired higher index of refraction.

The above model cannot represent a complete description of the opticalproperties of the water isotopes, because it neglects differences in themolecular energy levels among the isotopes. Therefore Table 2 representsan approximation. Better measurements of the refractive index of thewater isotopes are desirable to confirm or adjust these numbers. Thedifferences in the indices of refraction among the isotopes also causesome difference in the optical absorption of the different isotopes.TABLE 2 Isotope Density ρ (gram/cc) Index of Refraction H₂O 0.9956781.43664 D₂O 1.10315 1.49188 D₂ ¹⁷O 1.158308 1.52112 D₂ ¹⁸O 1.2134651.55103

The natural abundance of heavy water is about 0.015%. This causes achange of about 8 ppm in the refractive index from that of pure H₂O,possibly a significant difference. FIG. 6 shows a plot of the variationin refractive index with different concentrations of heavy water basedon calculations. Pure D₂O has a refractive index of about 1.492, whichis close to the refractive index of calcium fluoride (Ca₂F) of about1.50 at the wavelength of 193 nm. Therefore, heavy water may be able toreplace some Ca₂F for chromatic corrections in an optical systemcontaining optical elements formed of fused silica or the like. Table 3further lists the index of refraction and dispersion at 193 nm for fusedsilica, calcium fluoride, and different isotopes of water based oncalculations. The dispersion of the water isotopes has not been measuredat 193 nm. At longer wavelengths, the dispersion of D₂O is lower thanthat of H₂O. TABLE 3 Material Index of Refraction (n) Dispersion (dn/dλ)Fused Silica 1.5607 −0.00158 Ca₂F 1.502 −0.00099 H₂O 1.4355 −0.00199 D₂O1.49188 not measured at 193 nm D₂ ¹⁷O 1.52112 not measured at 193 nm D₂¹⁸O 1.55103 not measured at 193 nm

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reviewing the above description. Thereare many alternative ways of implementing the methods and apparatus ofthe invention.

1. A liquid used in a liquid immersion lithography apparatus, the liquid comprising one or more isotopes of a liquid, wherein an isotopic composition of the liquid has been adjusted to provide desired optical characteristics.
 2. The liquid according to claim 1, wherein the isotopic composition has been adjusted by adjusting amounts of a plurality of isotopes.
 3. The liquid according to claim 1, wherein the optical characteristics include a refractive index.
 4. The liquid according to claim 3, wherein the refractive index of the liquid is greater than about 1.45.
 5. The liquid according to claim 1, wherein the liquid comprises one or more isotopes of water.
 6. The liquid according to claim 1, wherein the isotopes include one or more heavy isotopes.
 7. The liquid according to claim 6, wherein the one or more heavy isotopes include deuterium.
 8. The liquid according to claim 1, wherein the liquid is enriched with heavy isotopes.
 9. The liquid according to claim 8, wherein a proportion of at least one heavy isotope is at least doubled in comparison with the natural isotope distribution of the liquid.
 10. The liquid according to claim 8, wherein the liquid is enriched with deuterium.
 11. The liquid according to claim 10, wherein more than 1% of the molecules contained in the liquid contain deuterium.
 12. The liquid according to claim 11, wherein more than 80% of the molecules contained in the liquid contain deuterium.
 13. The liquid according to claim 13, wherein more than 99% of the molecules contained in the liquid contain deuterium.
 14. The liquid according to claim 13, wherein more than 99 molar % of the liquid consists of heavy water. 